OLED (organic light-emitting diode) and OPV (organic photovoltaics) technology is emerging as an alternative to different types of illumination/recharging purposes. Collectively, OLEDs and OPVs are referenced to as organic optoelectronic devices. In general, an organic optoelectronic device comprises two electrodes between which an organic optoelectronic material is sandwiched.
In an OLED, the optoelectronic material is an electroluminescent material. When a current is made to flow between the electrodes, the organic electroluminescent material emits light.
In an OPV device, the optoelectronic material is an organic photovoltaic material, which collects photons and transforms them into negative and positive charges so as to produce a voltage between the electrodes.
Due to the flexible nature of the organic optoelectronic devices, they may advantageously be used in flexible applications, i.e. applications where the device may be bent during normal operation, or on curved surfaces, for example providing a curved display device or illumination system in the case of an OLED.
In this context, a drawback of at least the current technology is that one of the anode and cathode electrodes that sandwich the optoetectronic material is highly reflective in order to obtain high light utilization. Hence, the devices have a mirror like appearance, which is not desired in some applications. For example, the appearance of the OLED in the OFF-state is important, and different solutions have been proposed in order to improve it.
U.S. Pat. No. 6,501,218 to Duggal et al. describes a device structure for outdoor signs OLED technology. Here, an OLED which is patterned into a sign, such as a character or a number, is combined with a highly scattering, non-absorbing coating over the light-emitting OLED regions and a highly absorbing coating over the non-emitting regions. The result is a sign that can be viewed by virtue of the OLED light under low ambient light level conditions thanks to the combination of the highly scattering material forming the sign (character, number) and the highly absorbing coating forming the outline of the sign.
U.S. Pat. No. 6,501,218 discloses the use of a scattering enamel coating on top of the OLED. However, the enamels need to be sprayed onto a plastic film or glass slide, which is then transferred to the OLED device. There is a demand for a coating that can be directly applied to the OLED surfaces without the need for intermediate coating steps.
The enamel coating of U.S. Pat. No. 6,501,218 has the further disadvantage that it will crack or flake off when the substrate onto which it has been sprayed is stressed or bent.
Also, there is a high interest in obtaining large surfaces containing luminous patterns for decorative and informative purposes, and it would be desirable in many cases if the patterns were visible only when the OLED surface emits light.
Furthermore, there is an interest in providing OPV devices which can be made invisible to a user, for example so as not to disturb the visual appearance of the device which is provided with a voltage from the OPV.